For Project 2, flow cytometry will be used to determine whether platelet FXI is expressed normally in patients with type II FXI deficiency, and whether platelet FXI can substitute for plasma FXI in platelet activation. Flow cytometry will also be used to verify recombinant co-expression of GPIbalpha, GPIbbeta, GPIX and platelet FXI on the surface of CHO and L-cells. For Project 3, flow cytometry will be used to study the expression of GPIIB-IIIA in platelets, in response to several G-protein coupled receptors (ADP, PAF, thrombin, U46619) and cell permeable activations (A23187 and PKC activators). For Project 4, flow cytometry will be used to estimate fibrinogen activation by platelet agonists in normal platelets, platelets from patients with bleeding disorders, and A5 cells under various conditions.